If a temporally variable current Ip flows through a coil S, an electromagnetic field Hp builds up around said coil (see FIG. 1). If an electroconductive object O is located close to the coil, eddy currents Iw are induced on the surface thereof and in layers near to the surface, based on the principle of electromagnetic induction. The current density thereby decreases as the depth increases. The eddy currents Iw in turn result in an electromagnetic secondary field Hs, which is inverse to the primary field Hp and thus weakens it. This effect is also referred to as the eddy current effect and is illustrated in FIG. 1. As a result of the eddy current effect, the inductance Ls of the coil S loaded with sinusoidal current Ip decreases if the conductive object O is brought near the coil S. At the same time, the resistive component RS of the coil S increases. Thereby, LS and RS are the series-connected components of an equivalent electric circuit diagram for describing the coil impedance ZS of the coil S. The following applies: ZS=RS+jω·LS. However, if the electroconductive object O is made up of ferromagnetic materials, a magnetization of the object O occurs in addition to the eddy current effect. As a result, a further magnetic field Hm not to be ignored is formed (see FIG. 1) which is aligned with the primary field Hp and thus amplifies it. Said effect is referred to as the ferromagnetic effect and interferes with the eddy current effect. The ferromagnetic effect is normally more pronounced than the eddy current effect, and this tends to result in an increase in the inductance LS as well as an increase in the ohmic resistance Rs in the above-described equivalent circuit diagram of the coil S.
In the dissertation by Mallwitz R., “Analyse von Wirbelstromsignalen mit problem-angepassten Funktionen fër die zerstörungsfreie Materialprüfung”, University of Kassel, 2000, the prior art regarding the determination of material properties by means of the eddy current effect and the ferromagnetic effect is explained. According to this, eddy currents depend, on the one hand, on object properties such as the conductivity σ and the relative magnetic permeability μr and hence on physical material properties, but also on the surface condition and homogeneity in the eddy current range such as the size and shape of the object and, on the other hand, also on the excitation (magnetic primary field strength Hp and frequency) and the distance between objects.
Usually, eddy currents are induced in conductive materials by excitation coils, whereby the primary field is influenced by the resulting magnetic field. This influence leads, on the one hand, to a change in the impedance of the excitation coil, which can be measured (parametric principle), on the other hand, however, the induced voltage in an additional secondary coil changes, too (transformational principle). The excitation of the excitation coil with sinusoidal current is most common (single frequency method). In doing so, the coil is normally a component of an oscillating circuit, in most cases of a serial oscillating circuit. The current and voltages within the circuit change if a conductive object enters the effective range of the coil field.
Irrespective of the principle, the measurands can be measured either absolutely (absolute arrangement) or as differential signals to a reference sensor (differential arrangement).
The excitation of the magnetizing coil can also occur with pulsed currents (pulse induction method).
From the European patent application EP 1 347 311 A2, a process for sorting out metal objects of a particular material property, in particular nonferrous metals between themselves, using several pulse induction single probes which form the detector and pick up induction signals of the metal objects, is known, whereby changes in the induction signals emitted by the metal objects are evaluated for sorting out.
From DE 198 38 249 A1, a sorting plant for sorting metallic particles is known, by means of which it ought to be possible to determine which materials possess metal particles. Said plant consists of a detector comprising single probes in an array arrangement with a high spatial resolution. The particles pass through a magnetic alternating field in a frequency range from 0.1 MHz to 1.0 MHz. Eddy currents which reduce the magnetic field develop in the metallic particles. Those changes in the magnetic field are detected by magneto-resistive sensors and transformed into electronic signals which are analyzed. In practice, however, the sensitivity of this detector and the signal progression produced for this purpose are not sufficient for reliably sorting out, e.g., nonferrous metals between themselves. In addition, said detector is relatively expensive.
A device and a process by means of which it is possible to sort out metals of different colours, i.e., nonferrous metals, between themselves is indicated and described in DE 100 03 562 A1. A differentiation between the nonferrous metals becomes possible only by an optical sensor which is designed as a camera, whereby metal fragments have to be illuminated. Different air nozzles which are able to sort a bulk material stream in free fall are provided for sorting out. A second detector designed as a metal detector is actually also arranged, but said detector is unable to distinguish between different nonferrous metals. The metal detector is only able to distinguish between ferrous metals and nonferrous metals. In practice, it is therefore usable only in combination with the optical sensor. The optical sensor differentiates metals only according to their appearance, but not according to their electromagnetic properties.
Processes are also known in which an evaporation of materials to be separated is effected with a laser or a radioactive irradiation of those materials is performed. However, those processes are relatively expensive.
Furthermore, it is known and common to use so-called magnetic separators for presorting ferromagnetic materials from other composite materials, with a subsequent separation of all remaining metals by means of an eddy current separator.
The signal evaluation with single frequency methods is usually carried out by evaluating the peak value (“amplitude”) and/or the phase shift (“phase”) of the sinusoidal coil current toward the excitation potential of the coil, which excitation potential is also sinusoidal. Rectifier circuits with a subsequent maximum value storage, scanning-holding elements or synchronous rectifiers are generally used for the “amplitude measurement”. Phase-sensitive rectifiers (synchronous rectifiers) are routinely used for measuring the phase shift (see Austrian Patent AT 501669 or Tietze, U., Schenk, Ch.: “Halbleiter-Schaltungstechnik”, 11th newly revised edition (1899), p. 1058, 1212-1218).
In practice, however, the reliable acquisition of information from eddy current data which have been measured still is a problem of the material selective eddy current measuring technique. The reasons for this are to be found in the complexity of the underlying physical phenomena. Therefore, apart from few special cases, the signal forming process is currently not describable in a universally valid form and the mathematical inversion of this illustration is not possible (see Mallwitz, R. l.c.).